Discharge lamp sources apparatus and methods

ABSTRACT

Capillary discharge extreme ultraviolet lamp sources for EUV microlithography and other applications. The invention covers operating conditions for a pulsed capillary discharge lamp for EUVL and other applications such as resist exposure tools, microscopy, interferometry, metrology, biology and pathology. Techniques and processes are described to mitigate against capillary bore erosion, pressure pulse generation, and debris formation in capillary discharge-powered lamps operating in the EUV. Additional materials are described for constructing capillary discharge devices fore EUVL and related applications. Further, lamp designs and configurations are described for lamps using gasses and metal vapors as the radiating species.

This is a Divisional of application Ser. No. 09/001,696 filed Dec. 31,1997, now issued as U.S. Pat. No. 6,031,241. This invention is aContinuation-In-Part of U.S. application Ser. No. 08/815,283 filed onMar. 11, 1997, now issued as U.S. Pat. No. 5,963,616. This invention isfurther related to U.S. Pat. No. 5,499,282 to William T. Silfvast issuedon Mar. 12, 1996, which is incorporated by reference.

This invention relates to capillary discharges for use as imagingsources in Extreme Ultraviolet Lithography (EUVL) and other technologiessuch as EUV microscopy, interferometry, inspection, metrology, and thelike. The invention describes characteristics of sources that radiateintense light in the wavelength region between 10 and 14 nm. Theoperation of these sources can be determined by: (1) the gas or vaporpressure within the capillary which generates optimum emission flux; (2)the range of discharge currents at which sufficient radiation fluxoccurs but above which significant detrimental debris and bore erosionbegins; (3) the desired range of capillary bore sizes and lengths, somespecific gaseous media that radiate effectively in the capillarydischarges under the conditions described above, and (4) two specificconfigurations for housing the capillary discharge system.

BACKGROUND AND PRIOR ART

A commercially suitable Soft-X-Ray (or EUV) lithography facility willrequire an intense soft x-ray/EUV light source that can radiate within aspecific wavelength region of approximately 11 to 14 nm in the EUV partof the electromagnetic spectrum. This region is determined by thewavelength range over which high reflectivity multilayer coatings exist.The multilayer coatings can be used to manufacture mirrors which can beintegrated into EUVL stepper machines. Specifically, these coatings areeither Mo:Be multilayer reflective coatings (consisting of alternateultrathin layers of molybdenum and beryllium) which provide highreflectivity between 11.2 and 12.4 nm, or Mo:Si multilayer reflectivecoatings (consisting of alternate ultrathin layers of molybdenum andsilicon) which provide high reflectivity between 12.4 nm and 14 nm. Thusany intense EUV source emitting in the wavelength range of 11-14 nm maybe applicable to lithography. Two proposed EUV sources are synchrotronswhich generate synchrotron radiation and soft-x-ray emittinglaser-produced plasmas (LPP's). Synchrotron sources have the followingdrawbacks: the synchrotron and synchrotron support facilities cost up to$100 million or more: together they occupy a space of approximately1,000,000 cubic feel Such a volume is incompatible with a typicalmicrolithography fabrication line. Laser produced plasmas that have thenecessary wavelength and flux for a microlithography system require ahigh power laser to be focused onto a target material such thatsufficient plasma density can be produced to efficiently absorb theincident laser radiation. Laser produced plasmas have the followingdrawback: if a solid target material is used, the interaction of thefocused laser beam with the target produces an abundant quantity ofdebris which are ejected from the laser focal region in the form ofatoms, ions, and particulates. Such eject a can accumulate on andthereby damage the optics that are used in collecting the light emittedfrom the plasma The use of volatile target materials in LPP sources hasbeen successful in overcoming the debris problem. A volatile targetmaterial is simply a material which is unstable to evaporation in a roomtemperature vacuum, examples of these are liquefied or solidified gasessuch as oxygen or xenon, and also liquids such as water. For thesematerials any bulk mass not directly vaporized by the laser pulse willevaporate and will be subsequently pumped away. Thus the excess targetmaterial does not collect or condense on the optics.

Although such laser-produced plasma sources have been developed for EUVLusing oxygen and xenon as radiating species, there still exist twoprohibitive drawbacks for which no realistic scenarios of significantimprovement have been proposed. First, the total electrical efficiencyof such sources is of the order of only 0.005-0.025%. This results fromconsidering the multiplicative combination of the laser efficiency,which is of the order of 1-5%. and the conversion efficiency of laserlight to useful EUV radiation (within the reflectivity bandwidth of amultilayer-coated reflecting mirror) of approximately 0.5%. Second, thecost of a laser that would necessarily operate at repetition rates ofover 1 kHz would be a minimum of several million dollars.

To overcome the unique problems specific to the synchrotron sources andto the LPP sources we have invented a compact electrically producedintense capillary discharge plasma source which could be incorporatedinto an EUV lithography machine. Compared to synchrotrons and LPP's thissource would be significantly more efficient, compact, and of lower cost(both to manufacture and to operate). We envision that one of thesesources (along with all the necessary support equipment) would occupythe space of less than 10 cubic feet and would cost less than $ 100,000.One such embodiment of the proposed capillary discharge source was firstdescribed in U.S. Pat. No 5,499,282 by William T. Silfvast issued onMar. 12, 1996. That particular proposed source would operate in alithium vapor electrically excited to within specific ranges of plasmaelectron temperatures (10-20 eV) and electron densities (10¹⁶ to 10²¹cm⁻³) which are required for optimally operating a lithium vapordischarge lamp at 13.5 nm. That same patent also proposed soft-x-raylamps at wavelength of 7.6, 4.86, and 3.38 nm in beryllium, boron, andcarbon plasmas. These wavelengths, however, are not within the range ofwavelengths required for EUV lithography. Although that patent describedthe general features of these lamps, it did not give the specificdischarge current operating range that would minimize bore erosion andthe emission of debris from the lithium lamp, or the appropriate rangeof bore sizes for operating such a lamp. That patent did not mention theuse of other materials, such as atomic or molecular gases that could besuccessfully operated in the lamp configurations described in thatpatent; it naturally follows that neither could it have mentioned whatare the preferred operating pressure ranges of those gases that would besuitable for EUV lithography.

SUMMARY OF THE INVENTION

Although gaseous plasma discharge sources have been produced previouslyin many different kinds of gases for use as light sources and as lasergain media, none have been demonstrated to have sufficient flux atappropriate EUV wavelengths for operating a commercial EUV lithographymachine. Consequently the necessary plasma discharge current and gaspressure necessary to obtain the required flux for use in an EUVlithography system and/or related applications have not previously beenidentified and described. Likewise the required capillary discharge boresize range for EUV lithography, as well as some specific capillarydischarge configurations for use with gases and metal vapors have notbeen previously identified. The subject invention specifically indicatesthe range of gas pressures the range of discharge currents and/orcurrent densities under which debris ejected from the capillary isminimized, as well as some specific gases to be used under thoseconditions. Also described, are two specific discharge configurationsone of which is designed specifically for gases or vapors and requiresno vacuum window. We have termed this the “differentially pumpedcapillary discharge”. The other is designed specifically for metalvapors or liquid vapors. We have termed this the “heat pipe capillarydischarge.” It contains a wick which is located only beyond thedischarge capillary (unlike that described in U.S. Pat. No. 5,499,282 byWilliam T. Silfvast issued on Mar. 12, 1996, in which the wick islocated inside the capillary).

For purposes of definition of a capillary discharge, we are operating anelectrical current within an open channel of an insulating materialwhere the open channel is filled with a gas or vapor that allows forelectrical conduction within the capillary. The channel or capillary istypically of cylindrical shape with a diameter in the range of 0.5 mm to3 mm and a length varying from 0.5 mm to 10 mm. The ends of thecapillary are attached to conducting materials to serve as electricalinterfaces between the electrical current within the capillary and theelectrical current of the external circuit The capillary is filled witha gaseous medium that becomes ionized so as to provide a low resistancefor conduction of the electrical discharge current within the capillary.The electrical discharge current excites the gas or vapor within thecapillary which then provides the desired radiation in the spectralregion between 11 nm and 14 nm. The gas or vapor within the capillarywhen ionized by the discharge current thus acts as both an electricallyconducting medium and an EUV radiator.

The following objectives relate to capillary discharge sources operatingin the wavelength range of 11-14 nm and which, within that wavelengthregion, provide the necessary flux for their particular applications.The objectives relate to: debris formation, materials considerations,discharge geometry, and applications.

The first objective of the present invention is to define the necessarycapillary bore diameter and length ranges of a capillary dischargesource. These dimensions are determined by experimental evidence inwhich strong EUV emission was observed.

The second objective of the present invention is to define the currentsand current densities of operation of a capillary discharge sourcecontaining a gas or liquid vapor or metal vapor such that it will notproduce debris destructive to the optics for a duration of at least theindustry-defined Lifetime of those optics.

The third objective of the present invention is to describe a method ofpre-treating the capillary bore region so as to make it resistant toerosion or other changes in the capillary during subsequent normaloperation.

The fourth objective of the present invention is to define the necessaryoperating pressure range of a gas or metal vapor or liquid vapor orother atomic or molecular species present within the capillary of acapillary discharge source.

The fifth objective of the present invention is to describe the“differentially pumped capillary geometry.” This geometry obviates theneed for an EUV transmitting window which would provide a barrierbetween the vacuum within the condenser system and the gas required forthe source plasma emission.

The sixth objective of the present invention is to describe the “heatpipe capillary discharge” which contains a wick within a heat pipeconfiguration such that the wick is mounted only outside of thecapillary discharge region.

The seventh objective of the present invention is to describe variousmaterials which may be used in the “differentially pumped capillarydischarge” and/or the “heat pipe capillary discharge.”

The eighth objective of the present invention is to provide a capillarydischarge source for use in any of the following applications:microscopy, interferometry, metrology, biological imaging, pathology,alignment, resist exposure testing for microlithography, and extremeultraviolet lithography (EUVL).

A preferred method of operating a capillary discharge source in the 11nm to 14 nm wavelength region includes forming a discharge within acapillary source having a bore size of approximately 1 mm, and at leastone radiating gas, with a discharge current of approximately

2000 to approximately 10,000 amperes, and radiating selected wavelengthregions between approximately 11 to approximately 14 nm from thedischarge source.

The gases can include one radiating gas such as xenon or an oxygencontaining molecule to provide oxygen as the one radiating gas, eachhaving a pressure of approximately 0.1 to approximately 20 Torr.

The gas can include a metal vapor such as lithium, to radiate theselected wavelength regions and has a pressure of approximately 0.1 toapproximately 20 Torr.

Besides the radiating gas, a buffer gas can be used, wherein the totalpressure in the capillary can range from approximately 0.1 toapproximately 50 Torr. The use of multiple plural gases can includelithium radiating the selected wavelength region between approximately11 to approximately 14 nm, and helium as a buffer gas.

Another preferred method of operating a capillary discharge source inthe 11 nm to 14 nm wavelength region includes forming a discharge acrossa capillary source having a bore size diameter of approximately 0.5 toapproximately 3 mm, and a length of approximately 1 to approximately 10mm, and at least one radiating gas, with a discharge current density ofapproximately 250,000 to approximately 1,300,000 Amperes/cm², andradiating selected wavelength regions between approximately 11 toapproximately 14 nm from the discharge source.

A method of constructing the capillary discharge lamp source operatingin the ultraviolet wavelength region includes constructing a capillaryfrom an electrically insulating material, inserting at least one gaseousspecies in the capillary, wherein the capillary is used to generateultraviolet discharges. A metallic conductor such as molybdenum, Kovar,and stainless steel, can be used as electrodes on opposite sides of thecapillary. A nonconducting and the insulating material can be used suchas quartz, saphire, aluminum nitride, silicon carbide, and aluminaFurthermore, the capillary can be a segmented bore of alternatingconductive and nonconductive materials.

Another preferred embodiment of the discharge lamp source operating theultraviolet wavelength region can include a capillary, a first electrodeon one side of the capillary, a second electrode on a second side of thecapillary opposite to the first side, a pipe having a first end forsupporting the second electrode and a second end, a discharge portconnected to the second end of the pipe, a wick passing through the pipefrom the discharge port to a portion of the pipe adjacent to but notwithin the capillary having a lithium wetted mesh for operation as aheat pipe, and means for operating the capillary as a discharge sourcefor generating ultraviolet wavelengths signals.

Pre-processing techinques of the capillary discharge bore source is whenthe bore is used with an optical element that operates in theultraviolet region, prior to operating the source, in order to preventrupturing of the optical element or contaminating mirrors that receiveradiation, are disclosed. The pre-processing techniques include thesteps of pre-conditioning interior bore surface walls of a capillarydischarge source that operates in the ultraviolet region, and continuingthe pre-conditioning until a selected impulse value is reached.

The pre-processing technique can use a heat source, such as an excimerlaser, a Nd:Yag laser, and a Copper Vapor laser. The laser can befocussed within the bore, and operated at a focussed intensity in therange of approximately 10⁷ to approximately 10¹¹ Watts/cm².

Another version of the pre-processing technique has the selected valueless than approximately 20 Torr-μs, wherein initiating discharge currentdischarge pulses within the capillary with a second gas having apressure range of approximately 1 to approximately 20 Torr., and thepre-operation pulses are approximately 3000 pulses.

Further objects and advantages of this invention will be apparent fromthe following detailed description of a presently preferred embodimentwhich is illustrated schematically in the accompanying drawings.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows a spectrum of xenon in the 11 nm to 14 nm spectral regionproduced in a 6 mm long 1 mm bore size capillary discharge at variousdischarge currents. It can be seen that at discharge currents below3,500 Amperes or 450,000 Amperes/cm² the emission at 13.5 nm and at 11.4nm (the two wavelengths of particular usefulness) decreases to a valuethat is significantly below peak measured emission at higher currents.To allow for the possibility of other gaseous or metal vapor speciesbeing more efficient than xenon in radiating, we have thus defined aminimum current at which significant emission is produced in relation toa minimum discharge current within a 1 mm bore size capillary at 2,000Amperes or a minimum discharge current density when extrapolating toother capillary bore sizes of approximately 250,000 Amperes/cm².

FIG. 2A shows a spectrum of oxygen in the 11 nm to 14 nm spectral regionproduced in a 6 mm long 1 mm bore size capillary discharge.

FIG. 2B shows the emission intensity of oxygen at 13 nm at various gaspressures at a discharge current of 6,000 Amperes when operated withinthe constant pressure capillary discharge configuration shown in FIG.3A. It indicates that the emission at 13.5 nm and 11.4 nm continues toincrease with pressures up to 10 Torr.

FIG. 3A shows a uniform discharge capillary configuration of the novelinvention for gases.

FIG. 3B shows a differentially pumped capillary configuration of thenovel invention for gases.

FIG. 4 shows a spectrum of xenon in the 1 nm to 14 nm spectral regionproduced in a 6 mm long 1 mm bore size capillary discharge at variousgas pressures at a discharge current of 6,000 Amperes when operatedwithin the differentially pumped capillary configuration of FIG. 4Bindicating an optimum pressure at the high pressure end of the capillarybetween 0.5 and 1 Torr and showing the emission reduces to a non usefullevel at a pressure of 0.15 Torr. This information suggests that apressure range of 0.1-20 Torr is the range of suitable operation of acapillary discharge. The upper limit is determined from knowledge ofplasma generation at higher pressures for which plasma arcs typicallyform. substantially inhibiting proper plasma formation.

FIG. 5 shows a capillary discharge configuration of the novel inventionfor producing strong emission in the 11 nm to 14 nm spectral region forEUVL and related applications in metal vapors that consists of a heatpipe operation at one end of the capillary in which the heat pipe wickis located only in the region outside of the capillary discharge region.In using this configuration, rather than a configuration in which thewick is within the capillary (as described in U.S. Pat. No. 5,499,282) asignificant improvement is incorporated into the design: the electricaldischarge current might flow through the capillary only by ionizing andelectrically exciting the metal vapor

within the capillary. In contrast, using the previous design currentcould have been electrically conducted through the capillary by the wickitself, rather than by the vapors within the discharge bore. However,even though the wick is not located within the capillary, it will stillserve to continually replenish the metal vapor pressure in the capillarydischarge region over the operating lifetime of the capillary dischargesource.

FIG. 6 shows a graph of the relative amount of debris generation(ablated mass from the capillary bore region) as the discharge currentis increased within the capillary in a 1 mm bore by 6 mm long capillary.This graph suggests that in this situation for an aluminum nitridecapillary material, the current should be kept below 5500 Amperes. Thisrepresents a discharge current density of 637,000 Amperes/cm². There canbe other possible bore materials that have higher erosion resistance sowe have set an upper limit on the current density (to allow for othercapillary bore sizes of 1,300,000 Amperes/cm².

FIG. 7 shows a graph of the reduction in the impulse produced on theaxis of the capillary at a distance of approximately 10 cm beyond theend of the capillary as the number of pulses of discharge current areinitiated within the capillary as the number of discharge current pulsesare increased within the capillary. It is desirable to have this impulseminimized to prevent rupturing of a window or other optical element Thiscan be obtained either by subjecting the bore to a number ofpre-operation pulses (3000 for the conditions shown in FIG. 8) or byheat treating the capillary bore surface with a laser or other means ofheat treatment so as not to have a disruptive pressure pulse duringoperation that could possibly damage a window or other useful elementthat is located beyond the capillary region but in the path of theemitted radiation emerging from the capillary.

FIG. 8 shows an over view of laser heat treatment of the novel capillarybore invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Before explaining the disclosed embodiment of the present invention indetail it is to be understood that the invention is not limited in itsapplication to the details of the particular arrangement shown since theinvention is capable of other embodiments. Also, the terminology usedherein is for the purpose of description and not of limitation.

Operating Conditions for a Pulsed Capillary Discharge Lamp for ExtremeUltraviolet Lithography (EUVL) and Other Applications Such as ResistExposure Tools, Microscopy, Interferometry, Metrology, Biology andPatholgy

The pulsed capillary discharge lamp sources that can be used with theseoperating conditions can be those described in U.S. Pat. No. 5,499,282to Silfvast; and parent U.S. application Ser. No. 08/815,283 to Silfvastet al., which are both assigned to the same assignee as the subjectinvention and which are both incorporated by reference.

For purposes of clarification: the gaseous species excited within thecapillary can be any of the following: 1. a pure, 100%. concentration ofan atomic or molecular gas (which } may also include vaporized atomicand/or molecular materials) in either their neutral or ionized stats:acting as the radiating species; 2. a buffered gas mixture of an atomicor molecular gas or vapor in either its neutral form or ionized formwith a second atomic or molecular gas wherein the first gas or vaporserves as the radiating species and the second gas serves as thebuffering species. The buffered gas interacts with the discharge,thereby promoting effective operation which might include but is notrestricted to any of the following processes: generation of appropriateplasma conditions (such as temperature and density), mechanism foreither cooling the electrons and/or for cooling the system, and for, inthe case of a vapor emitter, preventing vapor diffusion throughout thesystem such that the lamp operates in either heat pipe mode or as a puremetal vapor cell.

An example of a metal vapor radiator useful in the subject invention isa lithium metal vapor operating at one or both of the followingwavelengths: 11.4 nm and 13.5 nm.

An example of a buffered metal vapor lamp useful in the subjectinvention is a lithium metal vapor heat pipe as indicated in FIG. 5,buffered by helium or other gas and operating at one or both of thefollowing wavelengths: 11.4 nm and 13.5 nm.

An example of a discharge source useful in the subject invention using apure atomic or molecular gas is an oxygen lamp which contains a 100%concentration of oxygen operating on one or more of the followingwavelengths in five times ionized oxygen: 17.3 nm, 15.0 nm, 13.0 nm, and11.6 nm, as shown in FIGS. 2A and 2B.

An example of a buffered gas mixture in a lamp useful in the subjectinvention is a first atomic or molecular gas with a second atomic ormolecular gas is in a lamp which consists of oxygen as the radiatingspecies, (operating on one or more of the following oxygen lines: 17.3nm, 15.0 nm, 13.0 nm, and 11.6 nm.) buffered by any second gas such asone of the noble gases(helium, neon, argon, krypton, and xenon).

The subject inventors observed intense oxygen emissions at approximately17.3, 15.0, 13.0 and 11.6 nm, wherein the peak intensity per unitwavelength of oxygen at 13.0 nm is greater than that of a tin laserproduced plasma at its peak intensity per unit wavelength. The peakemission at 17.3 nm has been observed to be three times higher than at13.0 nm. Experimental evidence we obtained in a 1 mm bore capillarydischarge as shown in FIGS. 2A and 2B for oxygen, and FIGS. 1 and 4 forxenon, suggests that gaseous radiators existing in partial pressuresfrom approximately tens of millitorr up to approximately 20 Torr canintensely emit in the EUV. The range of currents and the range ofpressures for operation will now be described.

(1) Current Ranges for operation

A lamp with a 1 mm capillary using any radiating species would operatewithin the following current ranges, whereby the minimum currentrepresents the smallest current at which the required flux for theselected application is obtained, and the maximum current is determinedby the current at which significant bore erosion begins to occur. Foraluminum nitride capillaries this is anywhere between approximately 2000to approximately 5500 Amperes; for silicon carbide capillaries betweenapproximately 2000 to approximately 10,000 Amperes. Larger or smallercapillary bore sizes can be used consistent with the above currentdensities; for aluminum nitride capillaries: approximately 250,000 toapproximately 700,000 Amperes per square centimeter; for silicon carbidecapillaries: approximately 250,000 to approximately 1,300,000 Amperesper square centimeter. Other ceramic capillary materials can be operatedin a range of currents from a minimum current density of approximately250,000 Amperes/cm² and a maximum current density which is determined bythat current density at which significant bore erosion occurs (asdetermined by debris tests indicating reduced emission from the lampafter approximately 10⁸ to approximately to approximately 10⁹ pulses orwindow damage).

(2) Range of Pressures for Operation

For a capillary discharge lamp the radiating species can exist in apartial pressure range anywhere from approximately 0.025 toapproximately 20 Torr, and a total pressure (radiator plus bufferpartial pressure) no greater than approximately 50 Torr.

Techniques and Processes to Mitigate Against Capillary Bore Erosion,Pressure Pulse Generation, and Debris Formation in CapillaryDischarge-Powered Lamps Operating in the Extreme Ultraviolet (EUV)

The capillary discharge lamp sources that can be used with thesetechniques and processes can be those described in U.S. Pat. No.5,499,282 to Silfvast; and parent U.S. application Ser. No. 08/815,283to Silfvast et al., which are both assigned to the same assignee as thesubject invention and which are both incorporated by reference

(A) Operational Ranges

Erosion in ceramic capillary bores is substantially reduced if theoperational current and current density are held to certain limits, andwill be described in reference to FIG. 6. The range of operationalcurrents in 1 mm capillary discharges is the following: for aluminumnitride capillaries, peak currents between approximately 2000 Amperesand approximately 5500 Amperes, and for silicon carbide capillaries,peak currents between approximately 2000 Amperes and approximately 10000Amperes. The range of current densities for discharges in any sizecapillary is the following: for aluminum nitride capillaries, peakcurrent densities between approximately 250,000 Amperes per squarecentimeter and approximately 700,000 Amperes per square centimeter, andfor silicon carbide capillaries, peak current densities betweenapproximately 250,000 Amperes per square centimeter and approximately1,300,000 Amperes per square centimeter.

(B) Preprocessing of the Insulator

Material emissions from discharges in ceramic capillary bores is notconstant over the life of the capillary and can be substantiallydecreased if, before the capillary is incorporated into a final lampassembly, it is seasoned by exposure to a number of discharge currentpulses, and will be described in reference to FIG. 7. From thesefigures, and analysis, the pre-treatment of capillary bores by passingdischarge current pulses in the operational ranges described above isnecessary to reduce discharge material emissions. Between approximately1 and approximately 10,000 discharge pulses(for example 3,000 pulsesusing conditions in paragraph (1) for Characteristics common to alldischarges . . . as described below, are required, and pulses aboveapproximately 10,000 are not relevant to the process of emissionmitigation.

Pretreatment by discharge or other heat-treatment affects structuralmorphology of the ceramic bore. The morphological changes in thecapillary bore wall are the essential causal factors resulting inmaterial emissions decrease, and that means other than discharges canbring about the salutory changes. These other means can include, but arenot limited to, laser drilling and laser heat treatment as shown in FIG.8.

(1) Characteristics Common to All Discharges in this Investigation ofBore Erosion.

Capacitor bank with a total capacitance of 0.18 μF(microfarads) ischarged to voltage and discharged across a 1 millimeter nominal diameterby 6.35 mm long capillary in ceramic, either aluminum nitride (AIN) orsilicon carbide (SiC). At 5 kV discharge voltage, the total storedenergy is 2.25 J. so 1-2 Joules per shot is typical across thecapillary. Repetition rate is variable up to a present maximum of 60 HzThe current-versus-time curve looks like a damped sinusoid with 460 nsfull width for the first half cycle. The second half cycle peak is about−0.5 times the first half cycle peak. All discharge processing pulseswere made with 10 Torr argon gas fill.

(2) Bore Erosion Data

Beginning with a virgin capillary, we fired 1000 shots at a given peakcurrent We microscopically analyzed the capillary bore before and aftereach set of shots. Microanalysis measures average bore diameter at thecapillary face and also at a point slightly (estimated approximately0.25 mm) inside the bore from the face, this for both thehigh-voltage-facing side and the ground-facing side of the capillary.Hence four diameter measurements are made at each peak current, whichare expressed as ablated mass amounts by assuming uniform wear down theentire length of the capillary (this is not always true). In some casesthe bore begins to close up at one end; this is expressed as negativeablated mass amounts.

Referring to the graph in FIG. 6, fifty milligrams ablated masscorresponds to a 33% diameter increase, or a 76% increase in borecross-sectional area. Below approximately 5 kA, aluminum nitridecapillaries show very little erosion. Extended discharge runs show boreerosion at the 0 to 6% level after 100,000 shots at 4 kA. Siliconcarbide capillaries do not exhibit erosion out to 10 kA peak current(1.27 MA/cm²).

FIG. 6 shows the stability of SiC capillaries even at the high peakcurrent of 7500 A. Some very slight filling in of the ground side boreaperture is evident in these data at 10,000 shots.

(3) Pressure Pulse Data

Starting with virgin capillaries, we measured the pressure impulse(time-integrated overpressure) generated by the discharge by measuringmechanical impulse delivered to a moveable detector. While we have nodata on the temporal form of the pressure wave from these measurements,an assumption is typically made that its extent is roughly that of thecurrent, i.e. about half to one microsecond. Data from AIN capillaries(FIG. 7) show that an almost two order of magnitude decrease in impulseoccurs over the first few thousand discharges. We call this the“break-in” or “seasoning” curve. Systematics suggest this is caused byvaporization of more volatile components in the capillary bore innerwall. Morphology changes are seen microscopically.

Early results with ultra-thin windows provided by Sandia National Labsplaced approximately 10 cm from the discharge show survivability from3.5 kA discharge pressure pulses, but failure when the current wasraised to 4 kA. However, this data as taken with unseasoned capillaries(around 1600 shots at less than 3 kA before the window test was tried).So that more extensive testing with seasoned capillaries can still bedone.

(4) Witness Plate Debris Data

Plastic debris-collecting slides (22 mm square, approximately 160 mgeach) were placed at approximately 5 and 10 cm from the discharge, withthe top edge of the 5 cm plate slightly below the bore centerline andthe 10 cm plate square to the bore centerline, hence partially shadowedby the 5 cm plate top edge. Weights before and after shot runs wererecorded, using a scale with 100 microgram resolution and approximate200 microgram reproducibility. Fogging observed was patterned, notuniform as would be expected for vapor diffusing. A clear shadow of thetop of the 5 cm plate is seen on the 10 cm plates for all fogged sets.The as-laid transparent film which fogs after sitting on the shelfsuggests oxidation of a very thin, perhaps metal, coating. No evidenceof particulate deposition was seen in the fogged material when viewedmicroscopically, down to the resolution limit of the optical microscope(estimated at 0.5 micrometers). Atomic Force Microscope imaging can bedone for future testing.

Additional Materials for Construction of Capillary Discharge Devices forEuvl and Related Applications

Any of the previous materials combinations claimed for a lithiumdischarge lamp can also be used in operating lamps that use othergaseous media as described above, as well as those described in U.S.Pat. No. 5,499,282 to Silfvast; and parent U.S. application Ser. No.08/815,283 to Silfvast et al., which are both assigned to the sameassignee as the subject invention and which are both incorporated byreference. These materials can be based on the following: anycombination of metallic, electrically conducting electrodes and ceramicor insulating capillaries wherein the thermal expansions of the metallicand ceramic materials are closely matched to ensure the mechanicalrobustness of the lamp at its operating temperature, and such that thematerials are resistant to damage or corrosion by the emitting gaseousspecies and the buffering gaseous species (if present). These includebut are not limited to molybdenum as the metallic conductor and eitheraluminum nitride, alumina or silicon carbide as the ceramic insulator(as described in U.S. Pat. No. 5,499,282 to Silfvast; and parent U.S.application Ser. No. 08/815,283 to Silfvast et al. for use withlithium). For an oxygen emitter/helium buffered system, the abovementioned materials combination can be used, but more conventional andeconomic material combinations can be used including but not limited toKovar metallic conductor and an alumina ceramic insulator.

Capillary Configurations With Uniform Discharge and DifferentiallyPumped Discharge

FIGS. 3A and 3B show two assemblies that utilize the capillary dischargeEUV source. FIG. 3A shows an arrangement which maintains a uniformconstant gas pressure along the length of the capillary discharge. FIG.3B shows a configuration which utilizes the capillary bore itself as asolid-angle limiting aperture, giving a wide divergence of emitted EUVradiation at the expense of creating a gas pressure gradient across thelength of the capillary.

FIG. 3A shows an arrangement for producing and detecting EUV radiationusing a capillary discharge source. Electrode 300 is charged to highvoltage; as well, gas is fed to the cavity region contained by thiselectrode. This gas will contain the EUV radiating species, and in thesimplest case, will be the radiating gas itself, such as but not limitedto xenon gas. A discharge 304 is initiated between electrodes 300 and306 which flows through and is contained by the capillary bore in theinsulator 302. The electrode 306 can be a separate conductor within theassembly which completes the circuit, or it can simply be the groundedbody of the lamp housing as shown.

A differential pumping port 308 is a plug of solid material with a longnarrow bore hole, such as but not limited to 1″ thick stainless steelwith a 1 mm diameter hole drilled there-through. The differentialpumping port interfaces to a region 310 of high vacuum(less thanapproximately 0.01 Torr). The impedance to gas flow caused by the longnarrow hole allows the maintenance of a substantial gas pressuregradient across the differential pumping port. As a result, the gaspressure along the capillary discharge 304 is kept very nearly constantwhile the EUV can be propagated 312, and detected and analyzed by aspectrograph detector 314, under a vacuum condition. The gas pressureprofile versus position in this assembly is plotted in 316. The basepressure P at the discharge 318, can be maintained anywhere in a usefulrange from approximately 0.1 to approximately 10 Torr by adjusting thegas feed rate to the electrode 300.

FIG. 3B shows as less constrained sources assembly. Electrode 350 can befed with gas and charged to high voltage, and a discharge 354 to groundelectrode 356 is contained by a capillary bore in insulator 352, all aswas the case in FIG. 3A for 300, 304, 306 and 302, respectively. In thisassembly, however, the capillary bore itself is used as the differentialpumping port and the capillary directly interfaces the high vacuumregion 358. The EUV emission 360 propagates in a much wider sold angleas shown. As a consequence, the gas pressure profile 362 shows agradient along the capillary bore. Base pressure P, 364 is here in therange of approximately 0.1 to approximately 50 Torr.

FIG. 3B shows the novel lamp configuration referred to as“differentially pumped capillary geometry” which allows a lamp that usesgases(as opposed to a lamp that operates with metal

vapors) to operate without a window between the gaseous region and theoptics that collects the radiation emitted from the lamp in the 11 nm to14 nm wavelength region. Because of the very strong absorption ofradiation in that wavelength region by all materials, including gasses,it is necessary in an EUV lithography system, as well as otherapplications, to operate the imaging system within a very low pressureenvironment having a pressure of less than approximately 0.01 Torr.Hence, a lamp would generally need a window to separate the region ofthe lamp operating in the 0.1 to 50 Torr. pressure region from the lowpressure region(less than approximately 0.01 Torr) of the imagingsystem. Our differentially pumped capillary geometry allows for theoperation of the lamp containing the radiating gas without the need ofsuch as window. In the operation of this lamp, the gas is inserted atthe opposite end of the discharge capillary from that where theradiation flux in the 11 nm to 14 nm radiation is collected. Thepressure at that end of the capillary would be in the range of fromapproximately 0.1 to approximately 50 Torr. depending upon theparticular gas and the desired emission characteristics of the lamp. Thegas is pumped through the capillary by having a vacuum pump accessibleto the opposite end of the capillary, the end where the radiation fluxbetween 11 nm and 14 nm is collected and used in the desired opticalsystem such as EUV lithography. As the gas is pumped through thedischarge capillary the pressure drops approximately linearly such thatit is at the necessary low pressure(less than approximately 0.01 Torr.)when it emerges from the capillary. The lamp is operated just like otherlamps that have a constant pressure over the length of the capillarybore region by initiating a pulsed discharge current within thecapillary. We have observed that there is sufficient pressure within thecapillary, even at the low pressure side, to produce the desiredemission form the lamp and yet the region beyond the lamp hassufficiently low pressure to allow for transmission of the radiationbetween 11 nm and 14 nm. The capillary itself acts as a retarding systemfor the gas as it flows through the capillary so that the usage of gasis at a very low rate. The gas can also be recycled back to the highpressure side for reuse.

Lamp Configuration Structures for Lamps Using Gases and for Using MetalVapors as the Radiating Species

FIG. 5 shows a novel lamp configuration that can operate in the heatpipe mode having a wick on the front (window) side of the lamp. FIG. 5shows a metal vapor heat pipe type lamp assembly suitable for generatingEUV radiation from lithium vapor. The electrode 500 is charged to highvoltage and contains in its cavity some pressure of lithium vapor 504and a source of lithium such as a few grams of lithium metal or liquidlithium. A discharge 506 is generated between this electrode and anelectrode completing the circuit, which can most simply be the groundedbody of the lamp housing 510. The discharge is contained in thecapillary bore 508 of the insulator 502. The plasma 508 will be ionizedlithium and will radiate 522 useful narrow line emissions in the EUV. Tomaintain the lithium vapor pressure requires the use of a heater 514.heat sink 516, wick 512. and buffer gas 520. This is the principle ofthe heat pipe. Heater 514 can be a commercial high temperature resistiveoven such as but not limited to a Lindberg model 50002. Heater 514maintains an equilibrium vapor pressure between the lithium source inelectrode 500 and the lithium vapor 504. Lithium vapor flowing outtoward the cooler region of the assembly condenses as liquid lithium onthe wick 512. Wick 512 can be a stainless steel woven wire mesh fabricwith approximately 30 lines per inch or finer, which is rolled into ahollow cylinder shape and placed in contact with the inside tube wallsof the heat pipe body 510. A temperature gradient across the wick ismaintained by a cooling collar such as but not limited to afew(approximately 2 to 7) turns of refrigerated fluid(such as but notlimited to chilled water) flowing through a coil of copper tubing andconductively contacting the heat pipe body 510 as shown. The temperaturegradient thus created along the wick causes liquid lithium which hascondensed on the wick to flow back toward the hotter region, to maintainthe lithium vapor pressure on the EUV output side of the capillary. Abuffer gas 520, such as but not limited to helium, is necessary for theoperation of the heat pipe. In unheated regions, the system-wide gaspressure equilibrium is maintained by this buffer gas. In the vicinityof the wick 512, there is a transition region 518, where there arepartial pressures of both lithium vapor and buffer gas. In this region,nearer the capillary, the lithium vapor dominates, and as thetemperature decreases in going outward, the partial pressure of thebuffer gas progressively increases. Pressures balance so that throughoutthe entire lamp assembly, the total pressure(sum of lithium vaporpressure and buffer gas pressure is a constant

The region adjacent to the capillary must be maintained at a temperatureequivalent to the temperature necessary to generate the desired lithiumvapor density within the capillary. This will establish a lithium metalvapor in that region of the pipe. This vapor will diffuse into thecapillary and rear electrode region, and will not condense there as longas these regions are maintained at a higher temperature. Thus within thecapillary region is established a lithium metal vapor pressureequivalent to the saturated vapor pressure of the wick region adjacentto the capillary. A discharge is struck between the two electrodes 10,30 such that the current passes through the ceramic capillary, excitingthe lithium vapor, and generating soft x-rays. A buffer gas establishesa transition region in the pipe, on the window side, beyond whichlithium vapor diffusion is sharply reduced.

The heatpipe mode of FIG. 5 differ from that shown in FIG. 4 of thelithium heat pipe of U.S. Pat. No. 5,499,282 primarily in the placementof the wick. In that description, the wick is shown placed within thecapillary itself and extending into the rear electrode region, oppositethe window. In contrast the modified lithium heat pipe of subjectinvention FIG. 6 has a mesh wick 40 only on the front (window) side 90of the lamp 1, extending up to, but not beyond the capillary 20,creating a more favorable environment for conduction through the lithiumvapor within the capillary 20.

The minimum capillary bore diameter will be pressure sensitive and ofsuch a dimension so as to insure that sufficient collisions of electronswith ions occur to produce excitation of radiating states before theelectrons collide with the capillary wall and are consequentlyde-energized. It will also be determined by the size below if it isdifficult to initiate a pulsed discharge current within the capillary.Such a minimum diameter is of the order of approximately 0.5 mm. Themaximum bore diameter is determined by the desire to keep the radiatingflux to a minimal size so as to make it more readily adaptable to acondenser system for imaging purposes and also to keep the total currentto a reasonable size and yet still provide the optimum current densitydesired. A reasonable maximum size would be on the order ofapproximately 3 mm. The minimum length of the bore should be no smallerthat the capillary bore diameter. The maximum bore length should besufficiently long to produce enough radiative flux for the selectedapplication but not overly long so as to waste input energy to produceradiation that cannot be used because of being too far removed from theoutput end of the capillary. From geometrical considerations associatedwith radiating output flux, the bore length should be no longer thanapproximately ten bore diameters. Pass the 10 diameter bore length wouldrestrict the radiation flux.

Pre-Processing the Capillary Bore to Mitigate Against Bore Erosion

Techniques for pre-processing the inner bore walls will now be describedin reference to both FIGS. 7 and 8. FIG. 7 shows a graph of thereduction in the impulse produced on the axis of the capillary at adistance of approximately 10 cm beyond the end of the capillary as thenumber of pulses of discharge current are initiated within the capillaryas the number of discharge current pulses are increased within thecapillary. It is desirable to have this impulse minimized to preventrupturing of a window or other optical element. This can be obtainedeither by subjecting the bore to a number of pre-operation pulses (3000for the conditions shown in FIG. 8) or by heat treating the capillarybore surface with a laser or other means of heat treatment so as not tohave a disruptive pressure pulse during operation that could possiblydamage a window or other useful element that is located beyond thecapillary region but in the path of the emitted radiation emerging fromthe capillary.

Lasers have been used successfully for machining, heat treating, weldingand the like. In the subject invention, the laser can be used to heattreat the region inside the capillary bore to make it more resistive toerosion. This treatment would occur by subjecting the surface of thecapillary bore region, as shown in one embodiment in FIG. 8 to one ormore pulses of high intensity laser radiation, in the intensity regionof approximately 10⁶ to approximately 10¹¹ W/cm². The laser radiationwould heat the entire bore region as it passes through the bore of thecapillary. In some instances the lens can be adjusted along the axis tofocus on different regions within the bore.

FIG. 8 shows an example of preparing the capillary bore. Experimentallyit has been discovered that gas pressure pulses emanating from thecapillary on firing the discharge can be substantially reduced inmagnitude by preliminarily firing the discharge a few thousand times.The effect is to drive all condensed volatile materials out from thecapillary bore walls. Alternatively, a heat treatment using high powerlaser radiation can be applied to the capillary before it is mated tothe lamp assembly. FIG. 8 shows a heat treatment technique. A high powerpulsed laser beam 800, such as one generated from a laser such as butnot limited to an excimer laser, a Nd:YAG laser, a copper vapor laser,carbon dioxide laser, and the like, sufficient to produce fluences onthe order of approximately 10⁸ W/cm² or higher at the capillary. Laserbeam 800 will locally shock heat the capillary walls to near the meltingpoint, is focussed by a converging lens 802 to a focal point 804proximate and axially concentric to the capillary bore. The laser beam800 would irradiate the bore region and produce sufficient heating tochange the material structure of the bore to make it more durable andsmooth than would be achieved by the process that formed the bore, suchas the drilling process. Depending on the bore material used, laserpulses up to and larger than 1,000 or more can be used to achieve therequired compensation change in the bore material. The concentratedlight diverging just past the focus is intercepted by the capillary borewalls of the insulator 806 to be used in the EUV lamp assembly. Providedthe F number of the lens is smaller than the length-to-diameter ratio ofthe capillary(approximately 6 or higher), most of the light will beintercepted by the bore and only a small fraction will pass through thebore. For complete coverage of the length of the capillary bore wall,the insulator can be translated axially and also flipped to present theopposite fact to the light.

While the invention has been described, disclosed, illustrated and shownin various terms of certain embodiments or modifications which it haspresumed in practice, the scope of the invention is not intended to be,nor should it be deemed to be, limited thereby and such othermodifications or embodiments as may be suggested by the teachings hereinare particularly reserved especially as they fall within the breadth andscope of the claims here appended.

We claim:
 1. A differentially pumped capillary discharge lamp sourceoperating in extreme ultraviolet (EUV) wavelength region, comprising: acapillary constructed from a nonconducting and an insulating material;at least one gaseous species inserted into a pressure input end of thecapillary at a pressure of approximately 0.1 Torr to approximately 50Torr, the capillary having a wide angle emitting end being opposite tothe pressure input end; a first electrode at the pressure input end ofthe capillary; a second electrode at the emitting end of the capillary,the first electrode and the second electrode causing an electricaldischarge and EUV discharges to be generated through the capillary tothe emitting end; and means for pumping the gaseous species from theemitting end of the capillary causing a pressure differential betweenthe pressure input end of the capillary and the emitting end of thecapillary, so as not to have the gaseous species present above aselected pressure beyond the emitting end of the capillary that couldabsorb the EUV discharges.
 2. The differentially pumped capillarydischarge lamp source of claim 1, wherein the gas is chosen from atleast one of: helium, neon, argon, krypton and xenon.
 3. Thedifferentially pumped capillary discharge lamp source of claim 1,wherein the means for pumping includes: a low pressure of less thanapproximately 0.1 Torr.
 4. The differentially pumped capillary dischargelamp source of claim 3, wherein the means for pumping includes: a lowpressure of less than approximately 0.01 Torr.
 5. A method of generatingradiating discharges from a differentially pumped capillary dischargelamp source, comprising the steps of: feeding a gas at a first pressureof at least approximately 0.1 Torr into a pressure input end of acapillary; causing an electrical radiating discharge to be generatedthrough the capillary to an emitting end of the capillary; and pumpingthe gas from the emitting end of the capillary to cause a pressuredifferential between the pressure input end of the capillary and theemitting end of the capillary, so as not to have enough of the gaspresent beyond the emitting end of the capillary that could absorb theradiating discharge.
 6. The method of generating radiating discharges ofclaim 5, further including the step of: forming the capillary from aninsulating and nonconducting material.
 7. The method of generatingradiating discharges of claim 5, wherein the step of pumping the gasincludes: a second pressure of less than approximately 0.1 Torr.
 8. Themethod of generating radiating discharges of claim 7, wherein the secondpressure is less than approximately 0.01 Torr.
 9. The method ofgenerating radiating discharges of claim 5, wherein the radiatingdischarge includes: emission in extreme ultraviolet (EUV) radiationrange.
 10. The method of generating radiating discharges of claim 5,wherein the pressure feeding into the capillary is between approximately0.1 Torr and approximately 50 Torr.
 11. The method of generatingradiating discharges of claim 5, wherein the emitting end includes: awide angle emitting end.
 12. A differentially pumped capillary dischargelamp source for generating radiating discharges, comprising; a capillaryformed from an insulating and nonconducting material, having electrodes;a pressure source having a first selected pressure at one end of thecapillary for pumping gas into the capillary; means for forming adischarge across the electrodes and forming radiating discharges throughthe capillary to an emitting region; and means for pumping the gas froma second end of the capillary in order to form a pressure differentialbetween the one end of the capillary and the second end of thecapillary, wherein the radiating discharges are generated from thesecond end of the capillary without substantial absorption from the gasbeyond the second end of the capillary.
 13. The differentially pumpedcapillary discharge lamp source of claim 12, wherein the first selectpressure in the pressure source further includes: a range betweenapproximately 0.1 Torr to approximately 50 Torr.
 14. The differentiallypumped capillary discharge lamp source of claim 12, wherein the gas ischosen from at least one of: helium, neon, argon, krypton and xenon. 15.The differentially pumped capillary discharge lamp source of claim 12,wherein the means for pumping includes: a low pressure of less thanapproximately 0.1 Torr in the emitting region.
 16. The differentiallypumped capillary discharge lamp source of claim 12, wherein the meansfor pumping includes: a low pressure of less than approximately 0.01Torr in the emitting region.
 17. The differentially pumped capillarydischarge lamp source of claim 12, wherein the radiating dischargesinclude: emission in extreme ultraviolet (EUV) radiation range.
 18. Thedifferentially pumped capillary discharge lamp source of claim 12,wherein the second end of the capillary includes: a wide angle emittingend.
 19. The differentially pumped capillary discharge lamp source ofclaim 12, wherein the first selected pressure is at least approximately0.1 Torr.
 20. A method of generating radiating discharges from adifferentially pumped capillary discharge lamp source, comprising thesteps of: feeding a gas having a first selected pressure into an inputend of a capillary; causing radiating discharges to be generated throughthe capillary to an emitting end of the capillary; and forming apressure differential between the input end of the capillary and theemitting end by having a low pressure region adjacent to the emittingend, the low pressure region having a second selected pressure, thefirst selected pressure being higher than the second selected pressure,so as not to have enough of the gas present beyond the emitting end ofthe capillary that could absorb the radiating discharges.